#include <fstream>
using namespace std;
int main()
{
    ifstream fin("carnati.in");
    ofstream fout("carnati.out");
    long n,c,t[2000],p[2000],i,pmax=0,G,news,old,j;
    fin>>n>>c;
    for (i=0; i<n; i++)
        fin>>t[i]>>p[i];
    for(i=0; i<n; i++)
    {
        old=0;
        //news=p[i]-c;
        //if(news>pmax)
        //    pmax=news;
        //old=news;
        for(j=0;j<n;j++)
        {
            if(p[j]>=p[i])
                G=p[i];
            else
                G=0;
            news=old-(t[j]-((j==0)?0:(t[j-1])))*c+G;
            if(news<G-c) news=G-c;
            if(news>pmax)
                pmax=news;
            old=news;
        }

    }
    fout<<pmax;
    fin.close();
    fout.close();
    return 0;
}